ECE466 - Tutorial Simple Delay and Throughput Analysis
Jan 14, 2016
ECE466 - Tutorial
Simple Delay and Throughput Analysis
ECE 466
Overview of Topis
• Delay Analysis• Throughput Analysis
ECE 466
Delays in a Switched Network
• How long does it take to send three packets from A to B in– A packet switching network– A circuit switching network
• Which factors determine the delay?
A B
Switch 1 Switch 2
Switch 3
ECE 466
Delays in a Switched Networks
• All links have a transmission rate of C=100 Megabits per second (Mbps)• We want to send 200 bytes of user data from A to B
– Max. packet size is L = 100 bytes (including 50 byte header)• Distance between any pair of hosts and routers is 50 kilometers (km)• Routing is fixed: A 1 2 B
A B
1 2
3
ECE 466
Transmission delay
• Transmission delay is the time it takes to transmit data on a link, and is determined by
– D: Amount of data– C: Max. transmission rate (“capacity”) of a link
• In packet networks, transmission delay accounts for headersExample:
Data X = 200 bytesTransmission rate C = 100 Mbps
Transmission delay in circuit switched networks:Packet size L = 100 bytesHeader size H = 50 bytes
Number of packets = Transmission delay in packet network Transmission delay of packet
• In packet-switched networks (=store-and-forward networks), the transmission delay is experienced at each node
• In circuit-switched networks, the transmission delay is experienced only at the first node
Mbps 100bytes/ 200 X/C TCStrans
4 H)– (L / X P X/C A/C * P TPS
trans
/C Tpkttrans L
ECE 466
Propagation delay
• Propagation delay Tprop is the time that a transmitted bit needs to travel from one end of a link to the other end
• Propagation delay is only dependent on the speed at which signals travel on the transmission medium and the length of the link
Speed of light: 3 · 108 meters/sec,
Speed in guided media (approx.): 2 · 108 meters/sec
• Roundtrip propagation delay is twice the end-to-end propagation delayExample:
Distance d = 50 km Propagation delay of a link
End-to-End Propagation delay
Roundtrip delay
sTT
sTTTTee
proprtprop
Bpropprop
Aprop
eeprop
15002
750sec25032
22112
s
mkmT linkprop
250sec1025
sec)/102/(505
8
ECE 466
• Processing delay of a packet includes time to lookup the routing table and to move packet over the switch fabric
• Queueing delay of a packet is the waiting time in the output buffers (input queue in some case)
• Circuit-switched networks do not have processing or queueing delay
Processing and Queueing Delay in Packet Switches
memory
outputqueues
inputqueues
switchfabric
concrete view abstract view
ECE 466
Processing and Queueing Delay in Packet Switches
• Processing delay Tproc of a packet is a constant delay at each switch
– No processing delay in circuit-switched networks
– Since Tproc is small it is often neglected
• Queueing delay Tqueue of a packet which arrives at time t consists of the transmission delay of packets that
1. are in the node at time t,and
2. go to the same output link,
3. and are transmitted earlier than the arrival at time t.
• Queueing delay is variable, i.e., it depends on the backlog in the node due to other traffic
• Variable queueing delay is what makes analysis of packet networks hard
ECE 466
Delays for connection setup and termination
• In circuit-switched and virtual-circuit packet networks, there is in addition a connection setup delay Tsetup and a connection termination delay Tdisconnect
• Setup delay consist of the roundtrip propagation delay and processing delay at each node:
Tsetup = Trtprop + N · Tproc-setup
where
N is the number of nodes
Tproc-setup is the per-node processing delay for setup
Tproc-setup is the per-node processing delay for setup
ECE 466
Delays for connection setup and termination
• The connection termination delay depends on the site that terminates the connection
• If the caller disconnects the connection, we have
Tdisconnect = Trtprop + N · Tproc-dicsonnect
• If the callee disconnects the connection, we get
Tdisconnect = Te2eprop + N · Tproc-dicsonnect
ECE 466
Delay in a Circuit Switched Network
DATA
A 1 1 B
Tsetup
Tdisconnect
Tproc-setup
Host A Host BSwitch 1 Switch 2
1ApropT
eepropT 2
Tproc-disconnect
CStransT
ECE 466
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
Delay with Datagram Packet Switching
Packet 1
Packet 2
Packet 3
A 1 2 B
Tproc
Host A Host BSwitch 1 Switch 2
PStransT
pkttransT
1ApropT
ECE 466
Pkt1
Pkt2
Pkt3
Pkt1
Pkt2
Pkt3
Delay with Virtual Circuit Packet Switching
Pkt1
Pkt2
Pkt3
A 1 2 B
Tsetup
Tdisconnect
Host A Host BNode 1 Node 2
Tproc-disconnect
1ApropT
Tproc-setupTproc
PStransT
pkttransT
ECE 466
Putting it all together
• Determine the delay for transmitting three packets in a datagram network
• We use superscripts to denote the delay at nodes
• Since
we get
3,332,22
1,112211
pkttransqueueproc
pkttransqueueproc
pkttransqueueproc
Bpropprop
Aprop
datagram
TTTTTT
TTTTTTDelay
1,1,1, pkttrans
pkttrans
pkttrans
pkttrans TTTT
321procprocprocproc TTTT
•Exercise: Do the calculations also for virtual circuit and circuit-switched network. Plug in the numbers and compare !
3212 3 queuequeuequeuepkttransproc
eeprop
datagram TTTTTTDelay
fixed variable